This invention relates to electrochemical methods for varying the viscosity of fluids and more particularly to reducing the viscosity of salt-containing aqueous fluids which may be used in drilling fluid, completion fluid, workover, fracturing and other oil and gas stimulation applications.
Clear brine fluids are preferred over fluids such as muds for completion, workover and specialized drilling applications due to their solids-free nature which helps minimize formation damage. When additional solids transport capacity into or out of the wellbore is required or even suspension of fluid loss additives within the brine itself, certain polymers are utilized to provide the extra suspension properties. Oil and gas well operations such as gravel packing, fluid displacement, drilling in and underreaming with fluid loss protection are some examples where this extra solids carrying capacity is required.
An important benefit of using a brine over a mud is that even when contaminated with solids, the brine can easily be cleaned up and reused. Typically, settling, centrifuges, screens and filters can be used to clean up nonviscosified brine fluids.
However, once viscosified, conventional solids removal equipment is ineffective in removing contaminating solids due to the complicating presence of the polymer viscosifier. With filters, for example, polymers can coat out on the cartridges or filter medium producing high pressure drops within short periods of time. In order to efficiently remove undesirable solids from viscosified brines, the viscosity due to the polymer must first be "broken" to reduce the viscosity to near the virgin brine viscosity before conventional solids removal practices can be employed.
Viscosity reduction in brines can be achieved by several classical methods each of which involve cumbersome handling procedures. These methods have been borrowed from the stimulation and fracturing industry and have been modified for the completion and workover market segment. This includes the addition of various bleaches (in either solution or powder form) such as NaOCl, LiOCl and Ca(OCl).sub.2 Lithium hypochlorite, for example, is used in combination with cyanuric acid which stabilizes and slowly releases hypochlorite ion. After adding bleach, the viscosified brine is frequently heated to between 140.degree. F. and 160.degree. F. in order to initiate the viscosity reduction which is difficult to implement in a field location.
Viscosified brines are currently being used for various drilling, completion and workover applications. Once used, the customary practice is to dispose of them by either injection on site into the casing annulus or injecting them into a disposal well. Both practices are expensive since they involve loss of the brine value, trucking and disposal charges.
Conventional methods to reduce the viscosity, or "breaking," fluids typically employ high temperatures, bleaching agents, acids, enzymes and/or strong oxidizing agents. These methods, however, generally require (1) high temperatures which may not be feasible; (2) dilution of a fluid's density; and/or (3) the addition of undesirable chemicals. A convenient method to suitably reduce viscosity of various fluids is desired.